1. Field of the Invention
This invention relates to speed controls, to magnetic tape speed controls, and in particular to a magnetic tape transport including a speed control servomechanism. More particularly, the invention deals with such a tape transport and speed control system well adapted for use in a digital tape cassette apparatus.
2. Description of the Prior Art
Magnetic tape transports can be broadly classified into the capstan drive type and the reel or hub drive type. In the former type, the tape is transported from one reel to another by means of a motor-driven capstan pressing the tape against a cooperating pinch roller. In order to attain constant tape speed, therefore, the actual tape speed may be sensed from the rotative speed of the capstan itself, and the capstan drive motor may be controlled correspondingly for constant speed rotation. Although comparatively easy to afford tape transportation at substantially constant speed, the capstan drive causes considerable tape wear and the accumulation of static charges in the system. Additionally, the capstan drive necessitates the provision of many moving parts, and even with utmost care paid for proper adjustment of the capstan and pinch roller, the system cannot control tape speed as well as desired.
Such disadvantages are absent from the reel drive. With this latter system, however, the driving of the reels at constant speed results in varying tape speed depending upon the amounts of the tape wound on the two reels. In order to attain constant tape speed, therefore, the rotative speeds of the reel motors must be varied in accordance with the varying tape amounts on the reels, as by means of a servomechanism utilizing a signal representative of actual tape speed.
Of the various devices heretofore suggested and used for sensing actual tape speed, a typical example is found in U.S. Pat. No. 3,297,266 to Wilburn L. Rumple, which teaches the use of a "timing wheel" attached to or included in one of paired guide rollers defining a tape path from reel to reel on an open-reel tape transport. U.S. Pat. No. 3,600,654 to Katsuya Yasutake also teaches the use of one of the guide rollers for the generation of a tape speed signal. Nowhere in these patents is disclosed, however, how the speed sensing means can be applied to cassette tape transports.
In the case of an open-reel tape transport, as in the cited U.S. patents, the tape is in sufficient frictional contact with the guide rollers, so that the tape speed can be accurately ascertained from the guide roller rotation. Proper frictional contact of a speed sensing roller with the tape is far more difficult to realize in a tape transport for use with digital or audio tape cassettes of the internationally accepted Philips type. Even if the frictional contact is somehow established therebetween, moreover, magnetic tape cassette apparatus of the reel or hub drive type require additional considerations to be paid for optimum tape transportation.